A fuel cell comprises an electrolyte layer and a pair of catalyst carrying electrodes (referred to as catalyst electrode layers) placed on either side of the electrolyte layer, and generates electricity through an electrochemical reaction between fuel fluid such as hydrogen or alcohol and oxidizer fluid such as oxygen or air, which are supplied to the corresponding catalyst electrode layers, with the aid of the catalyst. Depending on the electrolytic material used for the electrolyte layer, the fuel cell may be called as the phosphoric acid type, solid polymer type or molten carbonate type.
In particular, the solid polymer electrolyte (SPE) type fuel cell using an ion conducting resin membrane for the electrolyte layer is considered to be highly promising because of the possibility of compact design, low operating temperature (100° C. or lower), and high efficiency. The electrolyte layer and the catalyst electrode layers disposed thereon are sometimes referred to as a membrane-electrode assembly (MEA).
Typically, such a fuel cell further comprises a pair of diffusion layers placed on either side of the MEA and a pair of separators (or distribution plates) disposed on either outer side of the diffusion layers. The separators can be formed by etching a silicon substrate, for example, and formed with channels (or recesses) for defining a flow passage for a fuel fluid (e.g., hydrogen gas) or an oxidizer fluid (e.g., oxygen gas) in their surface facing the diffusion layers. The diffusion layers are provided to diffuse the fluids evenly over the catalyst electrode layers as well as to contact the catalyst electrode layers to thereby transmit electric potential of the electrode layers to outside, and typically formed of an electroconductive porous material such as a carbon paper or a carbon cloth. The combination of a catalyst electrode layer and a diffusion layer may be called a diffusion layer. Further, in order to prevent undesirable leakage of the fluids, seal members are disposed between the electrolyte layer and the separators so as to surround the diffusion layers. A fuel cell assembly is formed by stacking these component parts and thereafter applying a tightening force on them in the stacking direction by using a tightening structure comprising backing plates disposed on either outer side of the separators, for example, so that the adjacent component parts are closely pressed to each other. In order to prevent leakage of the fluids through an interface between the separator and the diffusion layer or prevent the increase in the contact resistance between the diffusion layer and the catalyst electrode layer while keeping an undesirably large pressure from being applied to each component part of the fuel cell assembly, the externally applied tightening pressure need be maintained at a suitable level.
The electrolyte may consist of a solid polymer electrolyte (SPE). However, the SPE can function as an ion conducting membrane only when impregnated with water, and the SPE when impregnated with water significantly increases its volume. The volume of the SPE can also change depending on the temperature. Such volume increase of the SPE can generate stress inside the fuel cell assembly 1. Therefore, when the externally applied tightening force is large, the pressure applied to the component parts may become excessively high, which can cause a problem such as breaking the seal members 17, 18. Controlling the pressure at a constant level would result in an undesirably complicated operation. Also, the large tightening force tends to necessitate a bulky tightening structure for generating such a force, which would increase the weight and volume of the fuel cell assembly.
In order to solve these problems, it is proposed in International Publication WO01/95405 to provide a fuel cell assembly comprising: an electrolyte layer having a grid frame provided with a plurality of through-holes that can be formed by etching a silicon substrate, for example, and electrolyte retained in the through-holes; and a pair of separators interposing the electrolyte layer therebetween, wherein the electrolyte is aligned with fluid passages (recesses) defined by the separators. In this fuel cell assembly, the electrolyte retained in the through-holes of the frames can bulge into the fluid passages, and thus the increase in the pressure between the component parts of the fuel cell assembly due to the expansion of the electrolyte can be significantly reduced.
The above publication also discloses to bond the grid frame and the separators by using an adhesive agent or anodic bonding, for example. In the anodic bonding, an electrode layer and a glass layer are formed on the surface of the grid frame to be bonded, and a similar electrode layer is formed on a surface of the separators to be bonded. Then, the grid frame and the separators are brought into close contact to each other and heated to about 400° C., at which sodium ions become highly mobile. In this state, a voltage is applied to the electrode layers so as to move ions. Since the solid electrolyte is weak to high temperature and could be damaged if heated to the temperature of 400° C., a heater is placed under the electrode layers of the grid frame and/or the separators to allow localized heating. The heaters may consist of polycrystalline silicon, for example.
However, such provision of heaters complicates the structure, and increases the manufacturing steps. The bonding strength achieved by adhesive agents can significantly decrease when water is generated and humidity is increased in an operating state of the fuel cell assembly, and thus it is difficult to maintain a sufficient bonding strength for an extended period of time.